1. Field of the invention
The present invention relates to a semiconductor substrate, more particularly, it relates to a semiconductor substrate having a silicon substrate, an intermediate semiconductor layer composed of compound semiconductor material and deposited on the silicon substrate and a superconducting thin film layer composed of a compound oxide and deposited on the intermediate semiconductor layer.
The superconducting thin film layer of the semiconductor substrate according to the present invention can be used as a wiring material for semiconductor circuits which are built in the silicon substrate and/or in the intermediate semiconductor layer and for fabricating Josephson junctions for Josephson devices.
The combination of the superconducting thin film and the silicon substrate and/or the intermediate semiconductor layer according to the present invention can be used for building superconducting devices such as superconducting transistors, a hot electron transistors or the like.
2. Description of the related art
One of the basic technologies for fabricating an integrated circuit on a silicon single crystal substrate is a metallization technology for forming patterned conductor lines which are used for interconnecting a variety of electronic elements which are fabricated by the other basic technology including an insulating layer forming stage, a patterning stage of the insulating layer and an impurity doping stage by means of thermal diffusion, ion implantation or the like. However, a part of the signal current passing through the metallic conductor lines is lost since the cross sectional area of the metallic wiring line become very fine in the case of integrated circuits built on the silicon substrate. Therefore, it is demanded to deliver the signal current without current loss.
It is also demanded to increase the transmission velocity of signal current on the conducting lines in addition to increase the operating velocity of the active elements and passive elements themselves in order to improve the net operating velocity of the integrated circuits. However, the improvement in the transmission velocity of signal current through the conventional metallic conductor lines is limited because of the energy loss in the conducting lines. Still more, the increment in the integration degree lead to increment in power consumption owing to Joule heat which is generated in the metallic conducting lines, in other words, the degree of integration in the integrated circuit is limited. Thus, there have been strong demand to use superconductors as a material for the conductor lines.
The superconductivity is a phenomenon which is explained to be a kind of phase changes of electrons under which the electric resistance become zero and the perfect diamagnetism is observed. However, the critical temperature "Tc" of superconductivity could not exceed 23.2K of Nb.sub.3 Ge which was the the highest Tc for the past ten years, so that they have not used as a wiring material for ICs.
The possibility of an existence of a new type of superconducting material having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189].
It had been known that certain ceramic materials of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba--Pb--Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba--Bi--Pb type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10K and hence usage of liquidized helium (boiling point of 4.2K) as cryogen is indispensable to realize superconductivity.
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides show such higher Tc as 30K which are extremely higher than known superconducting materials.
It was also reported that C. W. Chu et al. discovered, in the United States of America, another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90K in February 1987. Still other type new superconducting materials which were reported recently are a compound oxide of Bi--Sr--Ca--Cu--O system and Tl--Ba--Ca--Cu--O system which exhibit such high Tc as more than 100K and which are chemically much stable than the abovementioned YBCO type compound oxide or the like.
And hence, the possibility of utilization of high-temperature superconductors as the wiring material for the integrated circuits.
The high-temperature superconductors are also demanded in the electronic devices. A typical application of the superconductor for the electronic devices is Josephson device in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors. The tunnel junction type Josephson device is expected to be a high-speed and low-power consuming switching device owing to smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as a high sensitive sensor or detector for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a precise quantum phenomenon. Development of the superconducting devices such as high-speed logic units or zero power loss wiring materials is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption.
Several ideas or concepts of new devices which is constructed by a combination of semiconductor and superconductor such as a superconducting transistor or a hot electron transistor have been proposed (M. Heiblum et al. "Solid State Electronics" Vol. 24, No. 343-346, 1981), but have not yet realized in a form of actual device.
In order to realize the above-mentioned superconducting devices such as the superconducting transistor or hot electron transistor, it is indispensable to prepare such a semiconductor substrate that has a superconducting layer which are stratified or deposited homogeneously on a silicon substrate.
The present applicant already proposed several processes for preparing the thin films of the high-Tc superconductor on oxide substrate in the following patent applications: U.S. patent application Ser. No. 152,714 filed on Feb. 2, 1988, U.S. patent application Ser. No. 167,895 filed on Mar. 13, 1988, U.S. patent application Ser. No. 195,145 filed on May 18, 1988, U.S. patent application Ser. No. 195,147 filed on May 18 1988, U.S. patent application Ser. No. 200,206 filed on May 31, 1988 or the like.
An object of the present invention is to provide a novel semiconductor substrate having a superconducting layer composed of compound oxide deposited on a silicon substrate through an intermediate semiconductor layer.